This invention relates to an ion retardation method for separating mixtures of salts from aqueous solutions.
It is often desirable when two or more salts are dissolved in an aqueous medium to separate the salts in order to obtain at least one of the salts in relatively pure form. Such separation is especially difficult when the salts contain a common cation such as, for example, sodium hydroxide and sodium chloride. However, in many industries, it is critical to prepare sodium hydroxide solutions which have as little chloride ion as possible. For example, in the preparation of ion exchange resins to be used as condensate polishers in the nuclear power industry, it is essential to prepare such resins with the smallest possible amount of chloride ion as the presence of chloride ion leads to stress corrosion and other problems within the nuclear reactor. Accordingly, such resins are advantageously prepared in the hydroxide form, using sodium hydroxide solutions which are free of chloride ion.
It has long been known that such separation of salts may be achieved through an ion retardation process. In such process, an aqueous solution containing the salts is passed through a bed of an ion retardation resin which retards the passage of one salt therethrough in relation to the other. Thus, the eluate from the resin bed will comprise a first fraction which is rich in one salt and a second fraction which is rich in the other salt.
The ion retardation resin is a so-called "snake cage polyelectrolyte" as discussed in an article in Industrial and Engineering Chemistry, Vol. 49, No. 11, November 1957, pages 1812-1819, entitled "Preparation and Use of Snake Cage Polyelectrolytes" by M. J. Hatch, J. A. Dillon and H. P. Smith. Such snake cage polyelectrolytes comprise a crosslinked polymer which contains pendant strong base groups which polymer further contains a second polymer, usually linear, which contains a plurality of pendant weakly acidic groups. In such snake cage polyelectrolytes the strongly basic groups and weakly acidic groups become ionically associated with each other. However, due to an excess of strong base groups or steric factors, some of the strong base groups do not become associated with a weak acid group. The presence of these unassociated strong base groups leads to a reduction of the ability of the resin to separate salts. As a result, some "leakage" of the undesired salt into the eluant fraction containing the desired salt occurs.
Previous attempts to reduce the amount of such leakage have focused on reducing the amount of unassociated strong base groups (hereinafter referred to as .DELTA.C.sup.+) in the resin. See, for example, U.S. Pat. Nos. 4,154,801 and 4,235,717. Typically, this is achieved by employing a large excess of the acid group-containing polymer. Unfortunately, however, such ion retardation resins having very low .DELTA.C.sup.+ are difficult to prepare and in use still do not separate salts completely.
Accordingly, it would be desirable to provide a method for separating mixtures of salts in aqueous solutions whereby very clean, efficient separation of said salts is achieved.